The present invention relates to the projection field, and more specifically concerns screens used both for front and for rear projection. Front projection is the projection of an image onto one side of a screen which by convention will be called hereinafter the front of the screen, for displaying images on the front of a screen. In the current state of technology, this type of projection is done in a darkened room, the conventional example being projection onto glass bead cinema screens.
Rear projection is projection of the picture onto one side of a screen which will be called by convention hereinafter the back of the screen for displaying images on the other side of the screen, which by convention will be called the front of the screen. Such screens are notably used for large scale projection or so-called picture walls; these screens, when they have sufficient contrast, are used in a normally lit theatre. As the projector, conventional analog projectors such as those of the 3-tube type can be used or one can also use, as is done in the equipment currently marketed by the applicant, digital devices such as the digital Micro-Mirror devices sold by Texas Instruments known as the DMD. Rear projection screens can also be used in other applications, for example as a screen for filtering a collimated or slightly divergent light, i.e. with an angle of divergence less than or of the order of 20xc2x0. Such screens can be used in signs for highways, or as directional filters on cathode ray tubes.
The ideal properties of a rear projection screen are as follows:
good luminescence or transmitivitty, in other words an ability to transmit light forwardly of the screen so that projected pictures are effectively displayed to the public, and that they are not at all or only slightly reflected back to the projector or absorbed by the screen;
high light absorption in the front to back direction so that ambient light is not reflected towards the public at the same time as the light projected from the back;
good resolution, in other words the ability to distinguish two projected points that are close to each other; controlled directivity, in other words the possibility of controlling the solid angle within which the rays passing through the screen are delivered; from this point of view, one can generally define a gain for the screen by comparing its characteristics with those of a diffusing reflector screen formed from a magnesium oxide layer on a backing support.
The ideal properties of a projection screen are substantially the same:
an ability to reflect light projected onto the front of a screen towards the public so that projected pictures are effectively reflected towards the public and are not or only slightly absorbed by the screen;
good resolution, in other words the ability to distinguish two projected points that are close to each other;
good resolution, in other words the ability to distinguish two projected points that are close to each other; controlled directivity, in other words the possibility of controlling the solid angle within which the rays passing through the screen are delivered; from this point of view, one can generally define a gain for the screen by comparing its characteristics with those of a diffusing reflector screen formed from a magnesium oxide layer on a backing support.
In the current state-of-the-art, projection screens are only used in a darkened hall, and the behaviour of the screen as regards ambient light is not a property that is considered.
Conventionally, the nominal contrast of a rear projection screen is defined as the ratio L0/(lxc3x97R) between the light L0 delivered by the screen and the product of the light l incident on the screen and the reflection R of the screen. This definition applies both to projection as well as to rear projection. In the case of rear projection, a reflection that is too high of light in a backward sense decreases the contrast of a projected image and can prevent the screen being used other than in a darkened room; as the rear projector is a box, rear projection tolerates minimal ambient lighting unlike front projection. This obviously creates a problem for applications such as control rooms or outdoor applications such as for example projection in sports stadiums.
Various rear projection screens have been proposed. The oldest and simplest solution is to use a frosted glass screen. A screen formed by a plate of frosted glass with a pitted surface constituting a Lambert frosting, with isotropic light scattering: the transmissivity of such a screen is consequently 50% and its gain is 1. Its front-to-back reflection is of the order of 10% which makes the use of frosted glass difficult under ambient light conditions. The Stewart Film Screen Corporation offers improved frosted glass screens having an oval-type forward gain and non-isotropic forward light scattering. Transmissivity is still around 50% but in the direction of use of the screen, gain is better than unity. Briefly, these frosted glass screens have high resolution but low contrast, which is typically of the order of 10. The company HIP is proposing screens formed from a thin film diffuser on a transparent substrates on which black dots embedded in the diffusing thin-film are deposited; these black dots reduce light reflection and increase screen contrast; nevertheless they also lower transmissivity and lead to information loss. Transmissivity is of the order of 50% and contrast typically between 50 and 100.
It is also known, for television applications, to provide a screen with a lenticular structure. Such screens have a wavy structure in a horizontal direction, unvarying by translation in the vertical direction. The waviness allows spreading of light horizontally thereby increasing viewing angle in this direction. The inclusion of diffusing cores such as diffusing bubbles inside the material has also been proposed to ensure controlled scattering in the vertical direction as well as in the horizontal direction: viewing angle in the vertical direction remains reduced and is in any case linked to bubble concentration; the use of such bubbles decreases screen resolution. Maximum resolution is fairly low in view of the minimum size of the wavy patterns which is of the order of 0.3 mm. With a wavy pattern size of the order of 0.8 to 1 mm, such screens are generally used for video. For high resolution graphic applications, such screens pose local or whole screen moirxc3xa9 patterning problems.
Such a screen is disclosed in European patent application 0,241,986; in this document, a black matrix is deposited between the undulations in order to improve contrast; this black matrix has the disadvantage of absorbing a part of the information. Transmissivity of such screens is of the order of 55% and contrast around 100. Dai Nippon Printing and Philipps offer such screens. At the SIG 99 (Symposium of international display) held at San Josxc3xa9, Calif. from May 16 to 20, 1999, Dai Nippon Printing presented a new lenticular screen having a layer absorbing ambient light applied directly to the outer cylindrical surface of the undulations; claimed improvement over the previous product is as follows:
Screen contrast and brightness remain pretty average.
J. L. Tedesco et al, Holographic Diffusers for LCD 9-32, mentions, for rear projection applications, the use of a screen formed from a Fresnel lens, a conventional diffuser and a lens matrix. The Fresnel lens forms an image of the lens aperture in a mid portion of the image space. The diffuser provides a limited diffusion of the image in the vertical direction and the lens matrix ensures image spreading in the horizontal sense. At the SIG 99, Sarnoff Corporation presented a new improved black matrix lens-structure screen without however stating how the black matrix had been improved; contrast appears good at but brightness remains fairly average in view of a transmission which at the best is 60%.
Briefly such lens-structure screens have low resolution, fairly average brightness, poor horizontal directivity but pronounced vertical directivity and heigh contrast if the black matrix is present significantly; however, in this latter case, a deterioration of brightness is present.
Physical Optics Corporation is selling, under the DDS (digital display screen) brand screens intended for rear projection or for television or computer screens. The screens are formed on a tinted polycarbonate, polyester or acrylic support onto which a holographic diffuser is bonded. The holographic diffuser is of the type disclosed in U.S. Pat. No. 5,609,939, and allows viewing angle to be controlled in other words the solid angle within which the light projected onto the screen is transmitted. Such screens resolve the problem of directivity; nevertheless, the tinted acrylic screen proposed for rear projection applications has poor brightness in view of the transmissivity which is only around 50%. All in all, such screens have good resolution, effective control of directivity in the horizontal direction as well as vertically; contrast and transmissivity remain poor notably in view of the use of a tinted material incorporated into the body of the screen; the transmission of such material is too poor to ensure good brightness and yet again too high to ensure good contrast. A contrast of the order of 50 is habitually encountered. Other suppliers of transmissive holographic films are Denso (Japan) or the Institut National d""Optique (INO, Canada). Holographic films are supplied by Physical Optics Corporation or by the US company Krystal Holographics International Inc.
The above article by J M Tedesco et al proposes associating a holographic diffuser and a Fresnel lens to overcome the problems posed by lens matrixes. U.S. Pat. Nos. 5,781,344 and 5,563,738 disclose low reflectance filters of the type currently used by the applicant for rear projection products. These filters consist of a support, an opaque matrix and balls which are impressed into the opaque matrix so as to come into contact with the support. Light originating from the projector is focused by the balls, and only passes through the opaque matrix when it passes the point of contact between the balls and the support or close thereto. For adjusting optical properties of the filters, these documents suggest providing, above the opaque matrix, at the side of the balls, one or several supplementary layers, between the balls or above them. To improve contrast by decreasing the amount of light passing between the balls, it is proposed, in this document, to deposit an opaque layer above the opaque matrix. This layer can for example be created by depositing a pigment in powder form and then heating the filter until the pigment diffuses into the upper portion of the opaque matrix.
This filter has a high resolution in view of the small size of the balls and their closeness. Nevertheless, the filling ratio of the rear surface by the balls is hardly 70%, which reduces brightness. A transmissivity of the order of 50% and contrast of the order of 200 is typically achieved.
Mems Optical Corporation and RPC (USA) are proposing micro-lens arrays. The micro-lenses are obtained by lithography using ion etching as for example described in international application WO-A-9832590. The micro-lenses are 10-2000 microns in size and are arranged regularly in a circle on hexagons, squares or rectangles. Such micro-lenses are not used for rear projection applications.
The Polaroid Corporation presented, at SIG 99, a display dedicated exclusively to LCD (liquid crystal display) projection. This is a diffuser with a linear polarising film; in the LCD cell, a single linear polariser is not useful, crossed with the polariser of the display. Ambient light passes through the display polariser and is back-scattered by the diffuser and consequently is absorbed when again passing through the display polariser.
British patent application 389,611 discloses a projection screen the rear surface of which, i.e. the surface on which the projector light is thrown, is formed from a large number of focusing optical systems. These focus light originating from a source at infinity towards apertures provided in a black layer. It is suggested to form these apertures in the black layer by depositing a photographic film on the rear surface of the screen, exposing the screen and developing the photographic film. By bonding a transparent material having surface irregularities or a semi-transparent material on the front face of the screen above the black layer, the screen can be used in daylight. In one embodiment, the focusing optical systems of the rear surface are formed by super-positioning two arrays each comprising cylindrical lenses.
The use of a semi-transparent or cloudy material considerably decreases display contrast as a not-insignificant part of the ambient light (30-40%) is back-scattered towards the user. Using surface irregularities in a transparent material as the diffuser also reduces contrast. This results in the contrast of the proposed display in that document being below 100.
British patent application 1,440,016 discloses a projection screen of the same type. In that document, it is proposed to provide, on the front face of a focusing element support, a black layer and to provide apertures therein, and to provide a diffusing material in these apertures. There is nothing in that document that teaches how to form the diffusing material in the apertures of the black layer.
It is difficult for the screens proposed in these two documents to control display directivity, in other words the orientation of the light rays leaving the display. Additionally, these displays cannot provide good contrast at the same time as a front face having a shiny appearance. The screens disclosed in these documents additionally suffer from significant aberration.
U.S. Pat. No. 4,666,248 discloses a rear projection screen having a transparent layer and, on the back of the screen, an absorbent layer having apertures and a layer of anamorphic lenses. This document states that around 75% of the intermediate layer is absorbent. French patent application serial number 980,402 relates to a projection screen using transparency, in other words a rear projection screen. This screen has two sets of cylindrical lenses with offset axes or lenticular elements formed by the intersections of said cylindrical lenses. On the other face opposite the lenticular elements, an opaque layer is formed with apertures using a photographic printing process. The major portion of this layer is opaque. French patent application serial number 972,333 discloses a rear projection device; it is provided on one face with microscopic step elements, for example a Fresnel lens. The other face carries an opaque layer having apertures formed by a photographic process. French patent application serial number 959,731 discloses a rear projection screen. It is formed from a support layer, an absorbent layer and a plurality of small spheres embedded in the opaque layer; the teaching of this document is similar to that of U.S. Pat. Nos. 5,781,344 and 5,563,738.
The invention proposes a solution to the various problems of rear projection screens. It provides a screen having excellent rear-to-front transmissivity, and good front-to-back absorption; it consequently provides excellent contrast. Further, in one embodiment, directivity can be controlled; it also avoids moirxc3xa9 patterning effects brought about by surface periodicity. In one further embodiment, a screen is provided the front surface of which has a brilliant or shiny appearance, and which nevertheless has excellent contrast and controlled directivity.
More precisely, there is provided a screen comprising a support with focusing elements, and an opaque layer with apertures allowing the passage of the light focused by the focusing elements, said apertures making up less than 10% of the surface area of the opaque layer.
In one embodiment, the opaque layer is close to the focal points of the focusing elements.
Preferably, the apertures are not dot-shaped.
In one embodiment, the size of the apertures is comprised between 2 micrometers and 200 micrometers.
In a further embodiment, the apertures make up less than 5% of the total surface.
The screen has a contrast greater than 250, and preferably greater than 500.
According to a further embodiment, the focusing elements have a dimension comprised between 20 micrometers and one millimeter.
The transmissivity of the screen is preferably greater than 70%.
According to a further embodiment of the screen, the square (xcfx86holes/xcfx86focusing)2 of the ratio between aperture dimension xcfx86holes and focusing element dimension xcfx86focusing is less than or equal to 10%, preferably less than or equal to 5%.
According to a further embodiment, the focusing elements comprise lenticular elements, the apertures are in the form of a line and the ratio between line width and a distance between two adjacent lines is less than or equal to 10%, preferably less than or equal to 5%.
In a further embodiment of the screen, the filling ratio by focusing elements is greater than or equal to 90%.
The focusing elements can comprise microballs in which case the screen preferably has a transmissivity greater than or equal to 80%, particularly preferably greater than or equal to 85%.
The focusing elements can also comprise microlenses or lenticular elements in which case the screen preferably has a transmissivity greater than or equal to 90%, particularly preferably greater than or equal to 95%.
The screen can further comprise a diffuser adjacent to the opaque layer, preferably a diffuser controlling directivity.
The screen can have a spacer layer between the support and the diffuser, preferably of a thickness between a few microns and several tens of microns.
This diffuser preferably has an active surface directed towards the spacer layer. The screen can have a transparent plate adjacent to the diffuser and bonded thereto. The diffuser is preferably a holographic diffuser. The screen can comprise a reflector adjacent to the opaque layer. The reflector is preferably a reflector controlling directivity.
A method for producing a screen is also provided, comprising the steps of:
providing a support having a plurality of focusing elements, and a layered material adjacent to the points of focus of said focusing elements;
irradiating said material through said focusing elements;
forming, using the irradiated material, an opaque layer having apertures making up less than 10% of the surface area of said opaque layer.
According to one embodiment of the method, the focusing elements comprise microlenses, lenticular elements or microballs.
According to another embodiment of the method, the focusing elements comprise microballs and the method further comprises the formation of a second opaque layer between the microballs, prior to the irradiation step.
According to one embodiment of the method, the material is an opaque positive-going photosensitive resin, and the formation step comprises the development of the resin.
According to n embodiment of the method, the material is a material that can be destroyed by irradiation and the formation step is performed by destruction of material at the same time as the irradiation step.
According to another embodiment of the method, the material is a positive photographic material and the formation step comprises:
the development of this photographic material.
According to a further embodiment of the method, the material is a material able to be decolored by irradiation and the formation step is performed by material decoloration at the same time as the irradiation step.
According to yet a further embodiment the method further comprises the steps of:
forming, on the support or the opaque layer, a spacer layer with a thickness of from a few microns up to several tens of microns;
forming apertures in the spacer layer, in correspondence with the focal points of the focusing elements;
bonding a diffuser onto the spacer layer, an active face of the diffuser being directed towards the spacer layer.
A transparent plate can be applied to the diffuser by bonding.
There is further provided a screen having a contrast greater than 250, preferably greater than 500.
Further characteristics and advantages of the invention will become more clear from the description that follows of some embodiments provided by way of non limiting example and with reference to the attached drawings.